The long-term goal is to characterize and isolate human lymphocyte subpopulations and evaluate the function of these populations in the immune response of both normal and diseased individuals. Recent work under this project has shown that anti-HL-A alloantisera contain antibodies against antigens which are found primarily on B lymphocytes and which are associated with B lymphocyte Fc receptors. In addition it was found that the Fc receptors of B lymphocytes and UL cells differ both in their avidity for Ig complexes and in the fact that the Fc receptors of UL cells do not appear to be associated with "Ia-like" antigens. Human peripheral blood mononuclear cells cultured in vitro with the antigen KLH-TNP generated significant numbers (compared to controls without antigen) of plaque-forming cells measured with SRBC coated with staph protein A in the presence of rabbit anti-human Ig, but not when measured with SRBC or SRBC-TNP.